In the process of acquiring reflected and refracted seismic energy from the earth, listening and recording equipment is typically arranged in long lines to form an array with locations or nodes along the lines where geophones are connected to the lines at the nodes. It is fairly common to have more than one geophone connected to a node or digitizing box and not uncommon to have two, three, four or five and sometimes many more at each node. For example, there was a survey in the Middle East that used 288 geophones at each node. For the purposes of this discussion, a node will represent a controlling box that could be an autonomous node or a cable connected analog to digital conversion box in the case of a cable based recorder. Typically, the geophones are stabbed into the ground in the same formation or arrangement around each node. The data coming from all the geophones is combined at the node and digitized so as to provide data from the node without separately identifying what each individual geophone received. The data is separated by node so that each node is viewed as a single receiver point and all the receiver points form a large three dimensional array for receiving and recording seismic energy emanating from the earth.
An individual geophone typically includes a magnet and an electrically conductive coil where one or both are arranged to move vertically or horizontally in the case of a three component geophone along an axis, one relative to the other. As the magnet and coil move relative to one another, the magnet induces a small electrical current in the coil that can be measured and recorded. Typically, springs having very minimal resistance to movement are used to bias the moving elements into a central location or neutral position where vibrations from the earth cause the magnet to deflect from the neutral position. The spring strength is typically arranged to be very slight so that slight vibrations of the ground due to the arrival of a seismic wave will cause movement of the magnet within the coil.
For hydrocarbon prospecting, geophysicists are interested in data having a broad frequency spectrum. Low frequency energy is used for the analysis of rock densities and wave velocity through the substructures while high frequencies provide higher resolution on smaller scale substructures. Geophones for hydrocarbon prospecting are designed to accurately sense and record a broad frequency spectrum of seismic energy with magnets size and weights to be within tight tolerances along with precisely engineered springs to permit movement in response to a broad frequency array. Most geophones used for hydrocarbon prospecting are rated for 10 Hz and greater although data is acquired below 10 Hz that is used. However, data below 8 Hz is typically absent in the data record.